More feedback :)

Do not return nil 
Check the last lecture of the mooc

Did you read Pharo with Style?

I do not know if LinkedList is the one of the system used by Process but in that case use the alternate implementation. 


bfs: info "Breadth fisrt search for an information. If there is no key containing given infomration returns nil. If the info is found returns a node containing it" | queue | self isEmpty ifTrue: [ ^nil ]. queue := LinkedList new. queue addLast: self root. [ queue isNotEmpty ] whileTrue: [ | node | node := queue removeFirst. node key == info ifTrue: [ ^node ]. node leftChild isNotNil ifTrue: [ queue addLast: node leftChild ]. node rightChild isNotNil ifTrue: [ queue addLast: node rightChild ]. ]. ^nil.

=>

bfs: info "Breadth fisrt search for an information. If there is no key containing given infomration returns nil. If the info is found returns a node containing it" | queue | self isEmpty ifTrue: [ ^ self ]. queue := LinkedList new. queue addLast: self root. [ queue isNotEmpty ] whileTrue: [ | node | node := queue removeFirst. node key == info ifTrue: [ ^node ]. node leftChild isNotNil ifTrue: [ queue addLast: node leftChild ]. node rightChild isNotNil ifTrue: [ queue addLast: node rightChild ]. ]